1. Field
The invention relates generally to wireless communication systems, and more particularly to systems and methods for reducing the amount of feedback that is needed to select appropriate data rates for encoding data streams in order to maximize data throughput.
2. Background
Wireless communication systems may include multiple base stations and multiple mobile stations. At any given time, a particular base station may be in communication with one or more mobile stations. Communications from the base station to the mobile stations are often referred to as the forward link or down link, while communications from the mobile stations to the base station are referred to as the reverse link or uplink.
Data that is to be communicated between the base station and mobile station is typically encoded, transmitted by a transmitter (either in the base station or the mobile station,) received by a receiver (either in the mobile station or the base station,) and then decoded. The data is encoded at a data rate that is selected based upon the quality of the communication link. The better the link, the higher the data rate that can be used.
While the base station typically has the capacity to be able to increase the power at which data is transmitted and thereby increase the channel quality, this may not always be desirable. For instance, if the quality of the communication link is already sufficient to support an adequate data rate, increasing the power may simply increase the interference with other communications. Base stations therefore typically implement some sort of mechanism to control the power and data rates at which data is transmitted. This may, for example, involve measuring performance (e.g., signal-to-noise ratio, or SNR) at the mobile station, providing feedback on the performance to the base station, and changing the data rate at which the data is encoded and transmitted based on the measured performance.
One of the more recent advances in wireless communications has been the development of MIMO (multiple-input, multiple-output) systems. A MIMO system uses multiple transmit antennas and multiple receive antennas to establish multiple channels that can be spatially distinguished from each other. One of the problems that has been encountered in the development of communications using MIMO technology is the maximization of throughput for each of the MIMO channels and the amount of feedback that is necessary to maximize the throughput.
One approach (referred to as Per Antenna Rate Control, or PARC) requires that a separate SNR value be provided as feedback for each of the MIMO channels. This approach is not ideal because of the large amount of uplink resources that are required to provide SNRs for each of the channels. Another approach (referred to as Diagonal Bell Laboratories Layered Space Time Architecture, or D-BLAST) only requires a single SNR value as feedback, but requires the transmission of null signals before transmitting the sequence of encoded data blocks for a part of the MIMO channels. This results in an inefficient utilization of the channels. A third approach (referred to as Code Reuse Bell Laboratories Layered Space Time Architecture, or CR-BLAST) also requires only a single SNR value as feedback, but it uses a single common encoder to encode all the MIMO streams. As a result, it cannot take the advantage of successive interference cancellation (SIC) and individually optimized rate control. Unless it is incorporated with highly complex iterative demodulation and decoding, the performance of CR-BLAST becomes much poorer than the systems employing SIC and individually optimized rate control. It would therefore be desirable to provide systems and methods in which a reduced amount of feedback (e.g., less than separate SNRs for each of the channels) can be transmitted from the mobile station to the base station on the uplink, in which the utilization of the channels is not diminished by the transmission of null signals, and in which individual rate control and SIC can be applied.